Pyridine as nucleophile

Nucleophilic Trapping of Radical Cations. To investigate some of the properties of Mh radical cations these intermediates have been generated in two one-electron oxidant systems. The first contains iodine as oxidant and pyridine as nucleophile and solvent (8-10), while the second contains Mn(0Ac) in acetic acid (10,11). Studies with a number of PAH indicate that the formation of pyridinium-PAH or acetoxy-PAH by one-electron oxidation with Mn(0Ac)3 or iodine, respectively, is related to the ionization potential (IP) of the PAH. For PAH with relatively high IP, such as phenanthrene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene, no reaction occurs with these two oxidant systems. Another important factor influencing the specific reactivity of PAH radical cations with nucleophiles is localization of the positive charge at one or a few carbon atoms in the radical cation. 

Quantitative studies of Michael-type additions in aqueous solutions are relatively scarce. Recently the rate-determining steps of the Michael reaction were investigated with substituted pyridines as nucleophiles (Heo and Bunting, 1992). The uncatalyzed Michael reaction proceeds nicely under neutral conditions when water is used as solvent, without any catalyst. 

In the S l Ar and the S l Ar mechanisms, the aryl halide becomes reactive towards nucleophiles by loss of an electron or a halide anion. Another means of activation is protonation, followed by excitation. This mechanistic pathway has been proposed for the photohydroxydehalogenation of 1-chloroanthraquinones in 97% sulphuric acid734, which leads to high yields (60-70%) of 1-hydroxyanthraquinones. The yields of the photochemical replacement of chlorine by pyridine in 1- and 2-chloroanthraquinone are much lower (3-6%)735. The reactions were performed with pyridine as solvent and the products were converted into aminoanthraquinones by treatment with alkali. Pyridine as nucleophilic... 

Halopyridines undergo self-quaternization on standing while the less reactive 2-halo isomers do not. However, more is involved here than the relative reactivity at the ring-positions. The reaction rate will depend on the relative riucleophilicity of the attack-ing pyridine-nitrogens (4-chloropyridine is more basic) and on the much lower steric hindrance at the 4-position. Related to this self-quatemization are the reactions of pyridine and picolines as nucleophiles with 4-chloro- and 2-chloro-3-nitropyridines. The 4-isomer (289) is. again the more reactive by 10-30-fold (Table VII, p. 276). 

Acyl chlorides are highly reactive acylating agents and react very rapidly with alcohols and other nucleophiles. Preparative procedures often call for use of pyridine as a catalyst. Pyridine catalysis involves initial formation of an acyl pyridinium ion, which then reacts with the alcohol. Pyridine is a better nucleophile than the neutral alcohol, but the acyl pyridinium ion reacts more rapidly with the alcohol than the acyl chloride.103... 

Chloroethyl)polystyrenes and (iodoethyl)polystyrenes are each prepared from the alcohol by common reagents in a single step without complications, but one-pot procedures fail to produce completely pure bromide, which must be prepared from the tosylate by assisted halide exchange (57). The preparation of (toluenesulfonyloxyethyl)polystyrene itself, if performed in ice-cold pyridine as for the free analogue (64, 65), required a week to complete (67) if quaternary ammonium and other side-products (68) are to be avoided. In contrast, with non-nucleophilic diisopropylamine (69) as acid acceptor instead of pyridine, (hydroxyethyl)polystyrene and toluene-sulfonyl chloride need only be refluxed in carbon tetrachloride for a few hours to give the desired tosic ester as sole product in quantitative yield (57). 

It has to be assumed that these processes are occurring on the boundary between SN1(P) and SN2(P) mechanisms in whose transition states considerable P—0(—Ar) bond cleavage takes place. The lifetime of the resulting, more or less free metaphosphate anion 102 then depends upon the nucleophilicity of the surrounding solvent. With pyridine, for example, a very fast reaction occurs so that the overall process approaches an SN2 reaction. Acceleration of the reaction by amines such as 2,6-lutidine, which are disqualified from acting as nucleophiles by steric hindrance, or by solvents such as dioxane, whiche are presumably too... 

The first attempt to effect the asymmetric cw-dihydroxylation of olefins with osmium tetroxide was reported in 1980 by Hentges and Sharpless.54 Taking into consideration that the rate of osmium(VI) ester formation can be accelerated by nucleophilic ligands such as pyridine, Hentges and Sharpless used 1-2-(2-menthyl)-pyridine as a chiral ligand. However, the diols obtained in this way were of low enantiomeric excess (3-18% ee only). The low ee was attributed to the instability of the osmium tetroxide chiral pyridine complexes. As a result, the naturally occurring cinchona alkaloids quinine and quinidine were derived to dihydroquinine and dihydroquinidine acetate and were selected as chiral... 

A three-component domino reaction of isobutyral-dehyde, N-benzylidene aniline and pyridine-2-thiol as nucleophile gives access to 1,2,3,4-tetrahydroquino-lines as shown by Annunziata et al..161... 

The oxidation of A -phenylhydrazones in the presence of pyridine leads to the formation of 5-triazolo[4,3a]pyridinium salts hy attack of pyridine as a nucleophile on the nitrilimine intermediate (Scheme 54) [77]. 

The phosphitylation proceeded in the presence of a catalytic amount of 4-(dimethylamino)pyridine as a nucleophilic [65] activator and-iV AT-diiso-propylethylamine as hydrogen chloride scavenger (step d). The desired phosphoro-amidite was prepared in almost quantitative yield. 

A remarkable aspect of the rhenium oxo complex chemistry is the ability of terminal oxo functions to act as nucleophiles. This has been used to incorporate dioxo complexes such as [Re02(py)4] or [Re02(CN)4] between the negatively charged interlayers of silicates like hector-ite (pyridine complex) or between the positive layers of a hydrotalcite-like Mg/Al double hydroxide. " The same concept has been applied for the isolation of molecular units containing Re-O-M bridges where M is a Lewis acid. ... 

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